[1] We present a detailed 3-D P-wave velocity model obtained by first-arrival travel-time tomography with seismic refraction data in the segment boundary of the Sumatra subduction zone across Simeulue Island, and an image of the top of the subducted oceanic crust extracted from depth-migrated multi-channel seismic reflection profiles. We have picked P-wave first arrivals of the air-gun source seismic data recorded by local networks of ocean-bottom seismometers, and inverted the travel-times for a 3-D velocity model of the subduction zone. This velocity model shows an anomalous zone of intermediate velocities between those of oceanic crust and mantle that is associated with raised topography on the top of the oceanic crust. We interpret this feature as a thickened crustal zone in the subducting plate with compositional and topographic variations, providing a primary control on the upper plate structure and on the segmentation of the 2004 and 2005 earthquake ruptures. Citation: Tang, G.,

The variation of the seismic properties with frequency of a brine-saturated sandstone sample with a porosity of 22% was measured using a forced oscillation apparatus. Two pairs of orthogonal biaxial strain gauges were pasted at different locations at the length center of the sample. In the frequency range of 1-300 Hz, using these two pairs of strain gauges, we observed experimentally the same global flow but different local flows (squirt flow). Therefore, the observation of the local flow is influenced by the position of the strain gauges in contrast to the global flow. Indeed, local flow is strongly influenced by the microstructure (structure of the grain contacts and microcracks). In particular, we show that although the sample is homogeneous in terms of porosity and crack density, it is not the case in terms of crack aspect ratio, which may slightly vary along the sample. A 3D diffusion model coupled with a simple squirt model was built to further interpret the data. These results show that the wave-induced fluid flows occur at different scales and controls the dispersion and attenuation of saturated rocks.

SUMMARY In fully fluid-saturated rocks, two common phenomena are documented both experimentally and theoretically for frequency-dependent elastic moduli and attenuation, that is, the drained/undrained transition and the relaxed/unrelaxed transition. When investigating these transitions with the forced oscillation method in the laboratory, it is crucial to consider the boundary differences between the laboratory and the underground. A 1-D poroelastic numerical model was previously established to describe these differences and their effects; however, the boundary conditions used in the model are actually different from the real experiment case, thus leading to inaccurate predication of the measurement results in a laboratory. In this paper, we established a 3-D poroelastic numerical model with a new set of boundary conditions that better represent the experiment conditions. Furthermore, the 3-D poroelastic modelling results were compared with laboratory measurements under the same boundary conditions, showing a much better fit than the 1-D model. Therefore, the 3-D model provides a more accurate and reliable approach to understand the regimes and transitions of elastic modulus dispersion and attenuation, and thus has great importance in interpreting the measurements of frequency-dependent properties of rocks in the laboratory.

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